The cell membrane (sometimes called plasma membrane or plasmalema) forms the thin molecular surface of every cell. It is made up of lipids, proteins and some carbohydrates in a flexible, dynamic, and ever changing array. The plasma membrane acts as a selective barrier between the inside and the outside of the cell and controls the exchange of materials between the cytoplasm and the surrounding liquid. It is almost impermeable to certain ions and molecules, and the cell uses specialized transport mechanisms to move molecules from one side to another.
Phospholipids are the major srtuctural components of most membranes. These molecules form a bileyer (a double layer of material) on the surface of the cell with their long hydrophobic hydrocarbon chains pointing inward to the center of the bilayer and their hydrophilic phosphate groups facing outward. Within this bilayer of lipids float various kinds of proteins, rather like ships in a lipid sea. Some proteins remain on the surface and are called extrinsic proteins, whereas intrinsic proteins are partially submerged or extend through the phospholipid bilayer.
Protein and lipid constituents of membranes are not fixed in any other location, but they can move and locate themselves at different points on the cell surface as required. Some, having carbohydrates or lipids attached to them, are complex glycoprotein and glycolipid macromolecules that play roles in recognition between cells and acts as receptor for molecules such as hormones.The physical state of membranes is dynamic. For example, when a cell adds extra cholesterol to a membrane, this changes the fluidity and converts the cell membrane from a liquid like state to a more viscous gel like state. Components may be added or taken away as the cell charges, grows or becomes a specialist. Our modern picture of the cell membrane is a dynamic one of constant change, movement, modification and adaptation. To view a neat video about cell membrane structure and the fluid mosaic model click here. What is the mosaic model of the cell membrane?